Power factor measures the ratio of average power to the apparent power in an electrical load. Power factor ranges from a value of 0 (where the impedance of the load is purely reactive) to 1 (for a purely resistive load). In practice, the power factor of electrical devices ranges somewhere between 0 and 1, and the closer this value is to unity, the more efficiently energy is consumed by the device and the less power is wasted. Therefore, for consumers of electricity that employ highly reactive loads (e.g. electrical induction motors), it is crucial that steps be taken to adjust the power factor of their apparent load to improve performance and avoid wasting enormous amounts of power. For example, a mill that consumes 100 kW from a 220-V line with a power factor of 0.85 will require 118 kW of apparent power supplied, but if the power factor is improved to 0.95, the apparent power supplied drops to 105.3 kW. Many utility companies require such consumers to take affirmative steps to adjust power factor.
Large factories are not the only environments to benefit from improvement in power factor. AC motors are present in many different electrical appliances and equipment from compressors to elevators, and since they are usually inductive in their input impedance, they often present a less than desirable power factor rating, especially under light load conditions or during certain periods of load variance. To improve the power factor in AC motors, controllers have been developed and are generally known in the art. Examples, as discussed in more detail below, may be found in U.S. Pat. No. 4,459,528 (Nola), U.S. Pat. No. 4,266,177 (Nola), and U.S. Pat. No. 5,821,726 (Anderson), the disclosures of which are fully incorporated by reference herein for all purposes.
In general, the power factor mitigation approach taken by many AC motor controllers is accomplished by sensing the phase difference between the current and voltage phasors and then using a controller to adjust the actuation of thyristors in each AC motor phase to attempt to reduce the voltage and current phase lag. In an ideal implementation, if the phase between the current and voltage phasors can be brought to zero, the load looks resistive to the power supply, and therefore, the power factor would approach unity. While unity power factor is not entirely practically achievable, small improvements in power factor can make substantial differences in power consumption.
Many different approaches to improving power factor in electrical motors have been developed over the years. Power reduction systems for less than fully loaded induction motors wherein the phase angle between current and voltage (motor power factor) is controlled are already known in the art. In such systems, the motor power factor is controlled as a function of the difference between a commanded power factor signal and the operating power factor, through control of thyristors (e.g. a triac) connected to the motor. A controller developed by Frank Nola in 1977 is exemplary of this type of power reduction system.
In the Nola controller a phase lag signal is obtained by the circuitry. The phase lag signal is compared with a command phase lag signal representing a desired minimum power factor of operation. The resulting difference signal, a circuit error signal, is then used to control the on and off time of a triac in series with the winding of the induction motor to maintain motor operation at the selected power factor. This has the effect of reducing the power input to a less than fully loaded motor.
The principle of the Nola controller is to reduce the average voltage supplied to the motor when the motor is not operating at full rated load, by switching off the voltage for a portion of each half wave cycle. A typical induction motor operates most efficiently at rated load. For loads below rated load, the efficiency drops off. The effect of reducing the voltage causes the motor to be a smaller horsepower motor at smaller loads which in turn causes the motor to operate closer to peak efficiency.
To accomplish the lowering of voltage, it is necessary to know the load at any given moment. The one variable that is easy to measure and relates to the load on the motor is phase lag of the current to the voltage. Hence the Nola controller is in reality a phase lag controller. Phase is measured and compared to a desired phase in a classical closed loop feedback system. Inherent in closed loop control is the necessity for the control to be stable along with other dynamic requirements. This can only be accomplished with negative feedback.
The Nola Design is a closed loop control scheme. In closed loop control, system stability is determined by the dynamic characteristics of the device being controlled. In Nola's case, the motor's electrical responses to changes in voltage determine the stability of the closed loop system. It is necessary to compensate the controller output with a lag to maintain stability. In addition the closed loop gain of the system must be set with sufficient gain margin to maintain stability. These factors result in two limitations.
The first limitation is the necessary controller lag or compensation, which reduces the response of the system. This has been addressed in the Nola-type controller by the addition of circuitry for canceling this time lag during periods where the motors load suddenly increases and for providing improved response speed to a change from lightly loaded to full load conditions in order to prevent motor stalling or vibration, especially when the minimum power factor command setting is relatively high.
The second limitation of a closed loop control system of this type is not readily apparent to users of the controller. Two things have to be satisfied for stable closed loop control. First the feedback signal must be negative and second the gain must be below the point that the system goes unstable. The relationship of phase lag, the feedback signal to the motors load and voltage is fixed by the motor's electrical design. A closed-loop controller requires a certain control relationship between input and output to satisfy these stability conditions. As a result the motors energy saving is limited to considerably less than is theoretically possible. In addition, in order to set the controller to maximum energy savings, the set point has to be lowered in the field until the motor can no longer operate at the actual loads. This has the effect of putting the motor on the ragged edge of operation.
A typical motor would operate with the following conditions: If the controller is set to give full voltage at rated horsepower, the voltage and consequential power saving at no load will be on the order of 10%. However, in certain cases, up to 30% of the power could be saved. The controller accomplishes this by lowering the full load input voltage a fixed amount which results in the no load voltage being reduced further together with a reduced input voltage at full load. This works for applications that have motors over designed for the load conditions encountered as long as the motor is never required to provide full horsepower.
It would be an advance in the art to provide a fast responding controller that has the capability to improve the control of the phase lag in induction motors and hence the amount of energy saved. It would also be an advance to provide a controller that is capable of working with a broad variety of electrical appliances that contain induction motors thereby improving power factor and start-up characteristics. It would also be desirable to provide a power factor improving controller that is programmable and may be customized to particular loads and operating conditions. It would also be an advance to obtain a controller that permits full voltage operation at full horsepower and minimum voltage operation at no load, thereby eliminating the need for field adjustment.